The long-term goal of this research is to establish how parenteral lipid nutrition regulates hepatic metabolic function and alters the risk of liver disease in infants. Infants given total parenteral nutrition (TPN) have increased risk for metabolic liver diseases. Our preliminary studies in premature piglets show that TPN enriched with phytosterols and n-6 fatty acids (FA) induces hepatic cholestasis, steatosis and inflammation compared to enteral nutrition. TPN also suppresses plasma FGF19, a product of farnesoid X receptor (FXR), and treatment with an FXR-agonist reverses cholestasis in TPN-fed piglets. Our central hypotheses is that parenteral lipid nutrition devoid of phytosterols and enriched with n-3 vs. n-6 FA and will prevent the TPN- associated hepatic cholestasis and steatosis in premature TPN-fed piglets via activation of FXR. AIM 1 will test whether parenteral lipid nutrition devoid of phytosterols prevents hepatic metabolic dysfunction independent of lipid load TPN-fed premature pigs. We will use in vivo metabolic (13C-bile acid turnover and 13C-palmitate oxidation kinetics) and liver transcriptomic and metabolomic approaches to systematically quantify metabolic pathways in hepatic bile acid and lipid metabolism. RNAseq analysis and metabolomic profiling will be used to identify differential expression of potentially novel hepatic genes and gene networks. Aim 2 will test whether phytosterols present in parenteral soybean-lipid emulsions induce cholestasis and metabolic liver disease via antagonism of FXR receptor function. We will measure 13C-bile acid turnover kinetics, serum biochemical markers, liver histopathology, and tissue expression of FXR and FXR target genes, especially CYP7A1, involved in bile acid metabolism. We will use hepatocytes to measure expression of FXR activity and establish the specificity of the molecular mechanisms using individual phytosterols as well as an agonist and antagonist of FXR. AIM 3 will test whether intestinal FGF19 secretion stimulated by natural and novel FXR- selective bile-acids prevents TPN-induced cholestasis by feedback regulation of FXR and CYP7A1 signaling. We will measure 13C-bile acid turnover kinetics, serum markers, liver histopathology, and tissue expression of FXR, FGF19, CYP7A1 and target genes involved in bile acid metabolism. We will use piglet hepatocytes cultures to measure mRNA expression of FXR target genes and FGF receptor-4 signaling in the presence of FGF19, phytosterols and different the lipid emulsions. These studies will test novel mechanisms to establish whether phytosterols in parenteral lipid nutrition adversely affect hepatic metabolic function and disease in a clinically-relevant, neonata animal model. These studies in premature pigs are highly translational and will lead to new clinical practices in nutritional support and prevention of liver disease in infants.